Arc plasma guns are utilized for such purposes as thermal spraying which involves the heat softening of a heat fusible material, such as a metal or ceramic, and propelling the softened material in particulate form against a surface to be coated. The heated particles strike the surface and bond thereto. The heat fusible material is typically supplied to the plasma spray gun in the form of powder that is generally below 100 mesh U.S. standard screen size to about 5 microns.
In typical plasma systems an electric arc is created between a water cooled nozzle (anode) and a centrally located cathode. An inert gas passes through the electric arc and is excited thereby to temperatures of up to 15,000 degrees Centigrade. The plasma of at least partially ionized gas issuing from the nozzle resembles an open oxy-acetylene flame.
U.S. Pat. No. 2,960,594 (Thorpe) of a predecesessor-in-interest of the present assignee discloses a basic type of plasma gun. FIG. 1 thereof shows a rod shaped cathode 28 and an anode nozzle 32. The cathode is located coaxially in spaced relationship with the anode nozzle operable to maintain a plasma generating arc between the cathode tip and the anode nozzle. Plasma-forming gas is introduced into an annular space 40 surrounding the cathode.
Thorpe also depicts in FIG. 1 thereof the mounting of the cathode onto an electrode holder 3 which is threaded into the body of the gun so as to provide adjustment of the position of the cathode. As indicated at column 6, lines 17-24, initial striking of the arc is achieved by screwing the electrode body toward the nozzle and retracting it. An alternative method taught for starting the arc is by providing a high frequency source of current. After the arc is struck the same may be "suitably adjusted" by screwing electrode holder 3. It is also indicated that the tip of the electrode may be positioned at a distance away from the entrance of the nozzle. (Column 6, lines 64-66.) However, there is no teaching or suggestion in Thorpe of exactly what position of the cathode is suitable or how to determine such a position.
U.S. Pat. No. 3,627,965 (Zweig) similarly shows a plasma gun with a threaded cathode holder (FIG. 4) and suggests it may be used to alter the arcing gap. Zweig similarly gives no further enlightenment as to the use of the threaded holder. This and the Thorpe patent are representative of early constructions of plasma guns whereby cathode adjustments were for experimental or initial-setting purposes.
U.S. Pat. No. 3,145,287 (Siebein et al) of the present assignee describes a simplified plasma spray gun of a type that has been sold and used commercially for more than 25 years, for example as a METCO type 3MB gun sold by The Perkin-Elmer Corporation, Norwalk Conn. Such guns have a fixed positioning between the cathode and the nozzle anode, which generally has been quite satisfactory for defining a reliable operating arc voltage. Such voltage has a dependence on the exact axial spacing between the cathode and the anode, being proportional over a small distance range so that, for example, a shorter spacing produces a smaller voltage. Therefore, manufacturing tolerance variations as well as an effective increase in spacing due to electrode erosion result in minor variations in arc voltage from gun to gun and from time to time.
As mentioned above the variations are not significant for most applications. However, for certain applications such the plasma spray coating of certain precision gas turbine components, the voltage variations result in enough variation in energy and heating of the spray material to cause a problem in reliable production of the coated components. Also, frequently, a nozzle or even a cathode is prematurely replaced after a small amount of erosion causes an undesirable drift in voltage. In some instances gun parts are selectively assembled to compensate for accumulative manufacturing variations within practical tolerances.
Thus there is a need for precision adjustment of arc voltage, particularly to provide a constant, selected voltage. Such adjustment is even more particularly desired for a type of gun that otherwise has been proven for a specific application so that a full development program to develop spraying parameters for a new gun may be avoided.
Therefore, objects of the present invention are to provide an improved plasma gun apparatus with provision for precision adjustment of arc voltage, to provide for such adjustment in a practical manner, to provide such adjustment for a type of gun that otherwise has been suitable for use in a specific application, and to provide a wrench assembly for making precision voltage adjustments in a modified plasma gun. Another object is to provide a novel method for making voltage adjustments in such a gun. Further objects are to provide a predictable arc voltage and to extend useful life of cathode and anode components.